Hydrofluorocarbons (HFCs) and in particular hydrofluoroolefins (HFOs), such as 2,3,3,3-tetrafluoro-1-propene (HFO-1234yf), are compounds known for their properties of refrigerants and heat-exchange fluids, extinguishers, propellants, foaming agents, blowing agents, gaseous dielectrics, monomer or polymerization medium, support fluids, agents for abrasives, drying agents and fluids for energy production units. Unlike CFCs and HCFCs, which are potentially dangerous to the ozone layer, HFOs do not comprise chlorine and thus do not present a problem for the ozone layer.
1,2,3,3,3-Pentafluoropropene (HFO-1225ye) is a synthetic intermediate in the manufacture of 2,3,3,3-tetrafluoro-1-propene (HFO-1234yf).
The majority of the processes for the manufacture of hydrofluoroolefins involve a dehydrohalogenation reaction. Thus, the document WO 03/027051 describes a process for the manufacture of fluoroolefins of formula CF3CY═CXnHp, in which X and Y each represent a hydrogen atom or a halogen atom chosen from fluorine, chlorine, bromine or iodine and n and p are integers and can independently take the value zero, 1 or 2, provided that (n+p)=2, which comprises bringing a compound of formula CF3C(R1aR2b)C(R3cR4d), with R1, R2, R3 and R4 independently representing a hydrogen atom or a halogen atom chosen from fluorine, chlorine, bromine or iodine, provided that at least one of R1, R2, R3 and R4 is a halogen atom and that at least one hydrogen atom and one halogen atom are situated on adjacent carbon atoms, a and b being able independently to take the value zero, 1 or 2, provided that (a+b)=2, and c and d being able independently to take the value zero, 1, 2 or 3, provided that (c+d)=3, into contact with at least one alkali metal hydroxide in the presence of a phase transfer catalyst.
This document teaches, in example 2, that, in the absence of a phase transfer catalyst, there is no reaction when 1,1,1,3,3-pentafluoropropane (HFC-245fa) is brought into contact with a 50% by weight aqueous potassium hydroxide (KOH) solution at ambient temperature and under pressure for 24 hours.
In addition, this document suggests a reaction temperature of between −20° C. and 80° C. but does not recommend the use of alkaline earth metal hydroxides due to their limited solubility in water.
The document WO 2008/075017 discloses a dehydrohalogenation process which comprises bringing a hydro(halo)fluoroalkane into contact with a base, optionally in the presence of a solvent. It teaches that, in the absence of a solvent, the hydro(halo)fluoroalkane can be sent over the base under hot conditions or in the molten state.
This document illustrates the dehydrofluorination reaction of 1,1,1,2,3,3-hexafluoropropane (HFC-236ea) to give 1,2,3,3,3-pentafluoropropene (HFO-1225ye) at 150° C. in the presence of a 50% by weight aqueous KOH solution. In the absence of a phase transfer catalyst, the conversion after 3 and a half hours is 57.8% and the selectivity for HFO-1225ye is 52.4% (test 1). In the presence of a phase transfer catalyst, this conversion is achieved after only 2.5 hours and the selectivity is virtually unchanged (test 4). As indicated in table 2 of this document, it is necessary to use an organic solvent in order to increase the selectivity for HFO-1225ye.
WO 2007/056194 describes the preparation of HFO-1234yf by dehydrofluorination of 1,1,1,2,3-pentafluoropropane (HFC-245eb) either with an aqueous KOH solution or in the gas phase in the presence of a catalyst, in particular over a catalyst based on nickel, carbon or a combination of these.
A description is given, in the document WO 2008/030440, of an embodiment for the dehydrofluorination of 1,1,1,2,3-pentafluoropropane using a basic aqueous solution in the presence of a nonaqueous and non-alcoholic solvent in which the HFC-245eb is partially miscible.
The document Knunyants et al., Journal of the USSR Academy of Sciences, Chemistry Department, “Reactions of fluoro-olefins”, Report 13, “Catalytic hydrogenation of perfluoro-olefins”, 1960, clearly describes various chemical reactions on fluorinated compounds. This document describes the dehydrofluorination of 1,1,1,2,3,3-hexafluoropropane (236ea) by passing through a suspension of KOH powder in dibutyl ether, to produce 1,2,3,3,3-pentafluoro-1-propene (HFO-1225ye) with a yield of only 60%. This document also describes the dehydrofluorination of 1,1,1,2,3-pentafluoropropane (HFC-245eb) to give 2,3,3,3-tetrafluoro-1-propene (HFO-1234yf) by passing into a suspension of KOH powder in dibutyl ether with a yield of only 70%.
Finally, the paper by Sianèsi Dario et al. in Annali di Chimica, Societa Chimica Italiana, Rome, Italy, vol. 55, nos. 8-9, 1 Jan. 1965, pages 850-861, describes the dehydrofluorination of 1,1,1,2,3,3-hexafluoropropane using KOH.
The dehydrofluorination reactions as described above result, in addition to the desired hydrofluoroolefin compound, in the formation of water and potassium fluoride. Furthermore, the implementation of such a reaction continuously is not easy on an industrial scale as at least three phases (gas, liquid and solid) are involved and the removal of the potassium fluoride formed in a stoichiometric amount presents a problem.
The present invention provides a process for the continuous or semicontinuous manufacture of a (hydro)-fluoroolefin compound which makes it possible to overcome the abovementioned disadvantages. The reaction involved in the process is a gas/solid reaction.
A subject matter of the present invention is thus a process for the continuous or semicontinuous manufacture of a (hydro)fluoroolefin compound, comprising (i) bringing at least one compound comprising from 2 to 6 carbon atoms, at least two fluorine atoms and at least one hydrogen atom, provided that at least one hydrogen atom and one fluorine atom are situated on adjacent carbon atoms, into contact with a solid reactant comprising calcium hydroxide with coformation of calcium fluoride, a product which can be recovered in value.
The process according to the present invention preferably provides a (hydro)fluoroolefin compound comprising three carbon atoms, advantageously a (hydro)fluoroolefin compound represented by the formula (I)CF3CY═CXnHp  (I)in which Y represents a hydrogen or halogen atom chosen from fluorine, chlorine, bromine or iodine and X represents a halogen atom chosen from fluorine, chlorine, bromine or iodine; and n and p are integers and can independently take the value zero, 1 or 2, provided that (n+p)=2, by bringing a compound of formula CF3CYRCR′XnHp, in which X, Y, n and p have the same meanings as in the formula (I) and R represents a fluorine atom when R′ represents a hydrogen atom or R represents a hydrogen atom when R′ represents a fluorine atom, into contact with a solid reactant comprising calcium hydroxide.
The present invention is very particularly suitable for the manufacture of a compound of formula (Ia)CF3—CF═CHZ  (Ia)in which Z represents a hydrogen or fluorine atom, from a compound of formula CF3CFRCHR′Z, in which Z has the same meaning as in the formula (Ia) and R represents a fluorine atom when R′ represents a hydrogen atom or R represents a hydrogen atom when R′ represents a fluorine atom.
Thus, 2,3,3,3-tetrafluoropropene can be obtained by dehydrofluorination of 1,2,3,3,3-pentafluoropropane with a solid reactant comprising calcium hydroxide and/or 1,2,3,3,3-pentafluoropropene can be obtained by dehydrofluorination of 1,1,1,2,3,3-hexafluoropropane with a solid reactant comprising calcium hydroxide. The 1,2,3,3,3-pentafluoropropene can be in the cis and/or trans isomer form.
The present invention can in addition be used for the manufacture of 1,3,3,3-tetrafluoropropene by dehydrofluorination of 1,1,3,3,3-pentafluoropropane with a solid reactant comprising calcium hydroxide.
In the process according to the present invention, the solid reactant can comprise essentially calcium hydroxide or can comprise predominantly calcium hydroxide and other metal hydroxides, such as NaOH, KOH, Mg(OH)2 and Ba(OH)2.
Use may advantageously be made, as solid reactant, of soda lime, which is calcium hydroxide comprising a few percent by weight of NaOH and KOH.
The KOH content of the solid reactant is preferably between 0.1 and 5% by weight and advantageously between 2 and 4% by weight.
The NaOH content of the solid reactant is preferably between 0.1 and 5% by weight and advantageously between 1 and 3% by weight.
The solid reactant, comprising calcium hydroxide, can be used in the powder form or preferably in the granule form advantageously exhibiting a high porosity. A granule particle size of between 1 and 10 mm is preferred.
The moisture content of the solid reactant is preferably between 1 and 20% by weight, advantageously between 7 and 20% by weight and more particularly between 10 and 15% by weight.
Mention may be made, as example of solid reactant, of soda lime, in particular that sold by Grace under the Sodasorb HP brand.
The process according to the present invention is preferably carried out at a temperature of between 100 and 170° C., advantageously of between 120 and 150° C. A temperature of between 130 and 140° C. is particularly preferred. The dehydrofluorination reaction according to the process of the present invention can be carried out at atmospheric pressure but it is preferable to operate at a pressure in excess of atmospheric pressure, for example between 1 and 10 bar absolute. Advantageously, this pressure is between 5 and 10 bar absolute.
The reaction can be carried out according to the normal means used industrially to carry out a reaction between a gas and a solid, such as, for example, in a fluidized bed reactor with the solid reactant as a fine powder or else in a fixed bed reactor or also in a rotating reactor of the cement kiln type with the solid reactant in the porous granule form.
The process is preferably carried out semicontinuously in one or more tubular fixed bed reactors operating alternately, for example according to the technology for the treatment of gases with molecular sieves. After reaction, the solid reactant, partially or completely converted to calcium fluoride, is discharged from the reactor. The calcium fluoride, after optional separation of the unreacted calcium hydroxide, can be used as starting material in the manufacture of hydrofluoric acid.
The process according to the present invention can comprise the recycling of the unreacted reactants in the dehydrofluorination stage.